Colour Vision & the Rail Industry

1. Colour Vision & the Rail Industry Dr Catharine Chisholm c.m.chisholm@bradford.ac.uk

2. Acknowledgements • Rail Standards Safety Board • Dr Richard Graveling, Institute of Occupational Medicine

3. Background • Optometrist • Lecturer in Optometry and Vision Science • Anglia, City, Bradford • PhD • Effect of scattered light on visual performance post laser refractive surgery • Occupational vision research • Evidence-based visual standards • CAA, Home Office, DfT, HMPS, NATS, RSSB

4. Presentation outline • Current colour vision standards • Examination of safety critical colour tasks • Background on colour vision • The assessment of colour vision • Acquired colour defects • Relevance of colour tests to the rail industry

5. Current colour vision requirements • EU directive (2007/59/EC) Annex II • Normal colour vision: use of a recognised test, such as Ishihara, as well as another recognised test if required  • Recognition of colour signals: the test shall be based on recognition of single colours and not on relative differences

6. What does this mean in practice? • The vast majority of colour normals pass • Only the mildest red/green colour defectives fail • The tiny proportion of the population with blue /yellow defects will pass • Not examined by Ishihara • 1 in 13,000 • Current practice is safe • DDA?

7. Visual task analysis • RSSB Human Factors (RSSB, 2011) • Interviews • Site visits • Discussions with equipment suppliers • Areas of focus • Trackside signals • Indicator lights and displays • Electrical and telecommunications wiring

8. Safety critical colour tasks • Connotative - no redundancy related to positional cues or labelling • Serious consequences of failure • Spectral characteristics must be well defined • Hardware must meet standards across the network and over time

9. Safety critical task for train drivers and others • Colour light signals • Red • Yellow • Green • Defined in Railway Group Standard GK/RT0045 • Specified in BS 1376 1974

10. Colour light signals • Train drivers • Alerted to trackside signals by track-based alarm and AWS sunflower • Nevertheless, rapid identification may be needed so vision remains important • In addition, some trackside workers and others need to identify head and tail lights on trains and vehicles, to identify direction of travel • Red • White Specified in BS 1376, 1974

11. Environmental conditions • Conditions of reduced visibility • Remove positional cues • Reduce brightness (sighting distances) • Increase scattering of short wavelengths • Green may look yellow • Green, yellow, white will NOT be confused with red by a colour normal

12. CIE x,y 1931 • Any colour can be specified in terms of x,y coordinates on the CIE chromaticity diagram

13. Reading repeater indicators / diagnostic LEDs / other indicators • Train drivers, signallers, level crossing attendants, telecommunications workers etc. • Train cabs • All colour redundant • Positional and labelling cues • Back-up alarms

14. Signal control centres Lever controls

15. Signal control centres Lever controls: signal status lights No colour dependencies for lever-controlled signal

16. Signal control centres NX mimic displays Colour redundant other than signal status light (red / green)

17. NX signal status lights • No definition of colours to be used • No consistency between sites • At discretion of manufacturer • Lamps and lenses discolour with age Green Green!

18. Signal control centres Computer-based mimic displays - complex scene. Poor use of colour Route sectors: magenta /grey Lack of standardisation of specific colours

19. Signallers - summary • Lever control – colour not safety critical • Also, Bardic lamp and flags can be labelled • NX mimic board – signal status lights • No standardisation of colours used • Computer-based mimic – route sectors • No standardisation of colours used • Different categories of signaller? • Logistics? • Movement of individuals?

20. Distinguishing between different colour cables • Signalling and telecoms engineers • Wiring is colour coded • red, blue, yellow, green, slate, orange, white, purple, and black (telecommunications and signal wires) • Other codes also in use (RAL codes) • standard electrical colours (green/yellow, blue, brown) • Older electrical colours (red and black)

21. Distinguishing between different colour cables • No colour redundancy • Range of colour codes used by different manufacturer’s of equipment • Colours not specified and may vary from batch to batch • Not possible to define colour requirements for these roles

22. Colour Vision Colour is the visual effect caused by the spectral composition of light emitted, transmitted, or reflected by objects • The colour appearance of an object depends on: • The colour properties of the light source • The absorption/reflection properties of the object • The colour processing of the eye and brain

23. The cones <7% blue cones at fovea • 6 million • 3 types (40:20:1 R:G:B) • Concentrated at the fovea • Less sensitive than the rods • Good visual acuity • Excellent colour vision • Discriminate 7 million colours and shades Better colour discrimination foveally

24. Normal Colour Vision • Normal Trichromacy : All spectral hues can be matched with a mixture of 3 primaries, long wavelength (red), middle wavelength (green) and short wavelength (blue) lights. (L, M, and S sensitive cones) • Red - Green colour vision is stable with age • Short wavelength (blue) sensitivity varies reduces with age (>55 years)

26. Signal Yellow

27. Signal green

28. Achromatic brightness channel Nerves to brain L cone Photoreceptors Neurons in retina Rod Red / Green channel M cone Blue/Yellow channel Rod S cone Colour signal transmission

29. Higher colour processing

30. CIE x,y 1931 C.I.E Chromaticity Diagram 1931MacAdam ellipses All colours within an individual ellipse look identical to a normal trichromat

31. Colour defectives • Congenital (gene mutation) or acquired (damage to retina or visual pathway) • Cannot appreciate or discriminate between as many colours as a normal person • Colours which look different to people with normal colour vision, look the same and are confused (if there is no perceived lightness difference). Poor discrimination of specific hues.

32. Congenital colour deficiency • Present from birth and of constant severity and type • Same defect in each eye • More common in males than females • Red/green defects far more common than blue • Range in severity from very mild to ‘colour blind’ (monochromats) • Clinical tests are designed to look for common colour confusions associated with congenital defects • Normal visual function (except monochromats)

33. Congenital colour deficiency • 8% men • 0.5% women • Confuse colours of the same brightness • Distinguish fewer colours • Either one or more cone types • missing (dichromat) or • deficient (anomalous trichromat)

34. ‘Red’ ‘Green’ Anomalous trichromat 1 deficient cone type Dichromat 1 cone type absent Protanope 1% Severe Variable: mild to severe Protanomalous 1% Severe Deuteranope 1% Variable: mild to severe Deuteranomalous 5%

35. Just as common in males as females • The exact prevalence is unknown • Tritanopia ~1 in 13,000 • Tritanomalous trichomatism ~ 1 in 500? Congenital tritan defects (blue/yellow)

36. Complete Colour “Blindness” : Monochromatism / Congenital Achromatopsia • Rod Monochromats • No functioning cones • Vision very poor • Equal male:female • Prevalence ~ 1 in 35,000 • Cone Monochromats • Single cone response: • short-wave (blue) cones only • Hue discrimination in low light • Vision below average to poor • Extremely rare

37. Which colours are confused?

38. Green Green Yellow Yellow Blue-green Blue-green White White Red Red Blue Blue Purple Purple Violet Violet Severe ‘green’ defect Deuteranopia No red / green sensitivity Severe ‘red’ defect Protanopia No red / green sensitivity, red colours appear dim Which colours are confused?

39. Colour confusions for anomalous trichromats • Same colours confused as for dichromats but only more similar colours, closer together on the CIE diagram. • Confusion ellipses vary in size and include a range of desaturated colours depending on the severity of the colour defect • Severe anomalous trichromatism • isochromatic zones (as for dichromats) • Mild anomalous trichromatism • ellipses only slightly extended compared to normal trichromats

40. Green Green Yellow Yellow Blue-green Blue-green White White Red Red Blue Blue Purple Purple Violet Violet Protanomaly Reduced red / green sensitivity, red colours appear dim Deuteranomaly Reduced red / green sensitivity Isochromatic confusion lines

41. Colour confusions red / orange / yellow / green brown / green green / white red / white Protan (red) & deutan (green) blue-green / grey / red-purple red / black Protan (red) green / grey / blue-purple Deutan (green) Violet/ Grey /Yellow-green Red/Red-purple Dark blue/ Black Yellow/ White Tritan (blue)

42. Normal Deuteranope Protanope How colour deficients see William Stroudley’s painting of a steam engine ‘improved engine green’

43. Sensitivity to red lights • Those without a ‘L’ (red) cone have no retinal receptor that can respond to the far red end of the spectrum • Protanopes – don’t see the far red • Milder protanomalous - red seen as dim • “I find the intensity of the red light diminished. If I am a distance away from the light, the amplitude of light is too low for me to see at all and thus I can't determine if it is the top, middle or bottom light burning. There just are no lights! As I get closer to the light the intensity increases and I start to see the red light. To compensate I watch the other vehicles ahead and if they start to reduce speed I know there must be a red light ahead.”

44. Colour Vision Testing • Screening - Abnormal / normal • Ishihara (red-green), Hardy-Rand-Rittler • Grading - Mild defects pass, severe defects fail • Farnsworth D15, City Uni 2nd Ed. • Classification – protan, deutan or tritan • Most tests although some better than others • Vocational – pass/fail criteria based on task analysis • Practical colour matching • Colour recognition (lanterns)

45. Illumination • CIE Standard Source C (6700K) or Source D65 (6500K) • MacBeth Easel lamp (350-400lux) • Corrected daylight fluorescent • Appearance of pigment colours changes with the spectral content of the illuminant • Tungsten and fluorescent alter the appearance of colours • Some slight deuteranomalous trichromats will pass the test

46. Ishihara Pseudoisochromatic plates • Screening/identifying red-green colour deficiency only • Exploits isochromatic colour confusions with colour camouflage and luminance masking • Full version 38 plates, 25 numerals and 13 pathways, 24 plate common • Viewing at 66cm distance, 4 secs for each plate. Lighting critical • Randomise plates • Do not touch the plates • Protect from light and dust

47. Ishihara 24 plate pass/fail • Fail : 1 or more on plates 2-15 • 15% of normals fail • Fail: 2 or more on plates 2-15 • Sensitivity • Fails 99% of colour defectives • Specificity • Passes 94% of colour normals (Vingrys 1984) but 100% if examiner allows for misreadings (Birch 2001)

48. Ishihara 24 plate • Plate 1: demonstration • Plates 2-15 Transformation or vanishing digits • Do not use hidden digit (poor sensitivity) • Plates 16-17 classification plates • Correctly classifies as protan or deutan only 50% of the time

49. Hardy-Rand-Rittler • 4th edition Richmond HRR (2002) • 4 demonstration plates • 6 screening plates • 4 red/green • 2 blue/yellow • 14 classification plates • 10 red/green • 4 blue/yellow • ~classify severity and type

50. Performance of 4th Ed. HRR • Fail: two or more errors on the six screening plates • Sensitivity 100% • Specificity 97.5% • Fail: three or more errors from the six screening plates • Sensitivity 98% • Specificity rises to 100% (no misreadings) • Performance for detecting tritan defects is unknown